Electrically driven valve actuators are well known and have been used for many years to drive valves for controlling hydraulic and pneumatic flows in industrial process plants, for example in the operation of butterfly valves used in oil refineries. A known valve actuator, for example our Limitorque Model SMC 2005, comprises a three phase electric motor which drives a reduction gearing having an output for connection to a valve to drive its valve member. The reduction gearing and the motor are mounted in a heavy explosion proof cast metal housing. Mounted in a compartment within the housing is a set of electrical contactors which switch a three phase electrical supply to the motor so that the motor can be stopped, started and reversed. The contactors are operated by relay coils themselves driven from a single phase of the a.c. supply. The contactors include a mechanical interlock arrangement to prevent simultaneous connection of the supply phases for both forward and reverse running of the motor as this would provide a short circuit across the three phases of the supply.
Mounted on the housing is a switch arrangement for switching current to the relay coils, for stopping starting and reversing the motor. The reduction gearing drives a set of position switches which indicate predetermined positions in the range of travel of the valve member. The switches thus may operate to define a fully open, a fully closed and two intermediate positions of the valve member. Also, a torque limit is provided, the torque limit switch being operated when the motor torque applied to the valve member exceeds a given level, for example in response to a foreign object becoming jammed in the valve. The switches are arranged to switch a single phase of the a.c. supply and are interconnected in a manner to control operation of the relays that switch electrical power to the motor. Thus, for example, the motor is arranged to switch off or reverse in response to the fully open limit switch being operated. Other similar motor operating constraints can be set up utilising the switches.
A disadvantage of this arrangement of switches is that they operate at the supply voltage, typically 120 volts or more. As a result the contacts of the switches have to be of a heavy duty nature and in practice are coated in silver. Moreover, heaters are in general provided to prevent condensation from producing a short circuit across the contacts. Also, at the supply voltage, there is a risk of arcing at the switch contacts and consequently precautions have to be taken in case an explosive gas mixture builds up in the housing which would be ignited by the arc. The component parts of the housing are machined to a close tolerance in the region where they engage one another and one of the parts is provided with a flange which prevents a flame front established within the housing from working through the joint between the housing points, to outside the housing, so as to prevent an explosion or fire becoming established outside of the valve actuator housing. Another disadvantage of this known arrangement is that the switches, relays and their contacts are bulky and take up a significant amount of space in the housing.
In practice three separate housing compartments are provided to receive the switches and relays and each compartment needs to be provided with an explosion proof seal as aforesaid.